Assembly and method of attaching stub shaft to drum of axial compressor rotor shaft

ABSTRACT

An assembly and method for an axial compressor includes an axial shaft extending between an inlet casing and a discharge casing along the longitudinal length of a compressor housing. The axial shaft assembly includes a first stub shaft, a second stub shaft, and a hollow shaft drum having a longitudinal axis. The first stub shaft and the second stub shaft are coupled to opposing ends of the shaft drum along its longitudinal axis. A plurality of keys is disposed in a radial arrangement near the outer circumference of the first and second stub shafts and the shaft drum. A plurality of pins engage the plurality of keys, whereby the pins prevent the shaft drum from rotating axially relative to the first stub shaft and the second stub shaft and prevent the shaft drum from axially and radially separating from the stub shafts.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates, in general, to axial compressors and, more particularly, to a method of attaching a stub shaft to a drum of an axial flow compressor rotor shaft.

2. Description of the Related Art

Turbo machines, such as centrifugal flow compressors, axial flow compressors, and turbines are utilized in various industries. Axial flow compressors, in particular, have a widespread use in power stations, jet engine applications, gas turbines, and automotive applications. Axial flow compressors are also commonly used in large scale industrial applications, such as air separation plants and hot gas expanders used in the oil refinery industry.

Axial flow compressors generally include a rotating shaft coupled to a central drum having a number of airfoil blades circumferentially arranged and spaced apart in multiple rows along the axial length of the drum. The airfoil blades rotate between a corresponding number of stationary blades attached to a stationary tubular casing. The working fluid, such as air, enters and exits the axial compressor in the axial direction of the rotating shaft. Energy from the working fluid causes a relative motion of the rotating airfoil blades with respect to stationary blades. The cross-sectional area between the central drum and the casing decreases from an inlet end to a discharge end, such that the working fluid is compressed as it passes through the axial flow compressor.

In the manufacture of axial flow compressors, one or more solid stub shafts are operatively coupled to a hollow shaft drum by way of an interference connection, such as a rabbet fit. Due to the design of the stub shaft and the hollow drum, the two components tend to grow apart both radially and axially during compressor operation. The major change in dimension occurs in the radial direction. The magnitude of radial expansion of the stub shaft is usually smaller than the magnitude of radial expansion of the hollow drum, which reduces the effectiveness of the interference fit. In order to overcome this problem, prior art systems utilize long through-bolts that extend between the inlet and discharge ends of the stub shaft. The through-bolts extend in an axial direction within an annular clearance space between the stub shaft and the hollow shaft drum and prevent the shaft drum from radially growing apart from the stub shaft.

Within the prior art, U.S. Pat. No. 1,142,069 to Westinghouse discloses a marine turbine including a rotor element having a drum to which an impulse wheel is bolted by means of a plurality of bolts. The first shaft end is formed integrally with the impulse wheel while the second shaft end is formed integrally with the drum. FIGS. 1 and 3 of the Westinghouse patent illustrate that the bolts are circumferentially arranged about the impulse wheel.

U.S. Pat. No. 3,749,516 to Samurin et al. discloses a rotor structure for turbo machines having stub shafts fixedly secured to the ends of a through-bolt. A plurality of impellers is attached to the stub shafts. The stub shafts are threaded onto the through-bolt and compressed axially to form a rigid structure. A plurality of driving means in the form of pins is positioned intermediate to the impellers and between the impellers and the stub shafts. Radially disposed keys are inserted between the impellers and the stub shafts to engage the pins and provide a driving connection to the impeller assembly from the stub shafts.

U.S. Pat. No. 7,335,108 to Lin et al. discloses a composite drive shaft with captured end adapters. The drive shaft assembly includes a composite tube captured by end adapters. The end adapters are captured at the end portions of the composite tube. Each end adapter includes an adapter tube interface configured to engage the composite tube. A plurality of lugs protrudes radially outward from the adapter tube interface to engage the composite tube at its interior surface. The lugs function to transmit the torque from the end adapters to the composite tube and vice versa.

Numerous disadvantages are associated with the use of these conventional devices to prevent relative movement between the stub shaft and the shaft drum. In embodiments where through-bolts are used, the bolts are subject to harmonic imbalance and resonance as the axial flow compressor is brought up to operating speed because the bolts must be of substantial length to extend along the entire stub shaft length. High speeds at which stub shafts are usually rotated further complicate these problems. Additionally, the through-bolts are fastened under a very high pre-stretch condition, which tends to weaken the bolts as the axial flow compressor is operated in multiple cycles.

SUMMARY OF THE INVENTION

In view of the foregoing, a need exists for an assembly and method for attaching the stub shaft to the shaft drum of the axial flow compressor, whereby the need for through-bolts for preventing relative motion of the stub shaft to the shaft drum is eliminated. An additional need exists for providing an assembly and method for attaching the stub shaft to the shaft drum of the axial flow compressor, whereby the assembly and method lower manufacturing costs and reduce maintenance expenses.

According to one embodiment, a shaft assembly for an axial compressor includes a first stub shaft, a second stub shaft, and a hollow shaft drum having a longitudinal axis. The first stub shaft and the second stub shaft may be coupled to opposing ends of the shaft drum along its longitudinal axis. A plurality of keys may be disposed in a radial arrangement near the outer circumference of the first and second stub shafts and the shaft drum. A plurality of pins may engage the plurality of keys, wherein the pins prevent the shaft drum from rotating axially relative to the first stub shaft and the second stub shaft and prevent the shaft drum from axially and radially separating from the stub shafts.

According to another embodiment, the shaft assembly may further include a plurality of shoulder structures provided on the first stub shaft and the second stub shaft and a plurality of shoulder recesses provided at opposing ends of the shaft drum along its longitudinal axis. The plurality of shoulder structures may be operative for engaging the plurality of shoulder recesses to form a rabbet fit between the first and second stub shafts and the shaft drum for maintaining coaxial arrangement.

In accordance with yet another embodiment, each pin of the axial shaft assembly may be dimensioned to be larger than the corresponding key to allow for an interference fit between the pin and the key. Additionally, each key may include a slot formed on the first and second stub shafts and a corresponding slot formed on the shaft drum. The shaft drum may further include a plurality of recesses provided on its outer surface in axially-offset rows, each of the plurality of recesses operative for receiving a rotor blade.

In another embodiment of the axial shaft assembly, one or more through-bolts may extend axially between the first and second stub shafts through the shaft drum. The one or more through-bolts desirably provides a closing force to prevent relative movement between the first and second stub shafts and the shaft drum. Each through-bolt may include threaded ends and a nut may be provided at each threaded end to tighten the through-bolt and couple the first and second stub shafts to the shaft drum.

According to another embodiment, an axial compressor may be provided to include a housing having an inlet casing provided at one end and a discharge casing provided at an opposing end and an axial shaft assembly provided inside the housing. The axial shaft assembly may extend between the inlet casing and the discharge casing along the longitudinal length of the housing. A plurality of stator blades may extend radially inward from an inner surface of the housing and a plurality of rotor blades may extend radially outward from the outer surface of the axial shaft assembly. The axial shaft assembly may further include a first stub shaft, a second stub shaft, and a hollow shaft drum having a longitudinal axis. The first stub shaft and the second stub shaft may be coupled to opposing ends of the shaft drum along its longitudinal axis. A plurality of keys may be disposed in a radial arrangement near the outer circumference of the first and second stub shafts and the shaft drum. Additionally, a plurality of pins may engage the plurality of keys, wherein the pins prevent the shaft drum from rotating axially relative to the first stub shaft and the second stub shaft and prevent the shaft drum from axially and radially separating from the stub shafts.

According to a further embodiment, a method of attaching stub shafts to a shaft drum of an axial compressor may include the steps of providing an axial shaft assembly having a first stub shaft, a second stub shaft, and a hollow shaft drum having a longitudinal axis. The first stub shaft and the second stub shaft are desirably coupled to opposing ends of the shaft drum along its longitudinal axis. The method may additionally include the step of providing a plurality of keys disposed in a radial arrangement near the outer circumference of the first and second stub shafts and the shaft drum. Furthermore, the method may include the step of providing a plurality of pins engaging the plurality of keys, wherein the pins prevent the shaft drum from rotating axially relative to the first stub shaft and the second stub shaft and prevent the shaft drum from axially and radially separating from the stub shafts.

In another embodiment, the method may further include the steps of providing a plurality of shoulder structures provided on the first stub shaft and the second stub shaft and providing a plurality of shoulder recesses provided at opposing ends of the shaft drum along its longitudinal axis. Desirably, the plurality of shoulder structures are operative for engaging the plurality of shoulder recesses to form a rabbet fit between the first and second stub shafts and the shaft drum for maintaining coaxial arrangement. Each pin may be dimensioned to be larger than the corresponding key to allow for an interference fit between the pin and the key. Furthermore, each key may include a slot formed on the first and second stub shafts and a corresponding slot formed on the shaft drum.

In accordance with another embodiment, the method of assembling stub shafts to a shaft drum of an axial compressor may include the step of providing one or more through-bolts extending axially between the first and second stub shafts through the shaft drum. The one or more through-bolts may be operative for providing a closing force to prevent relative movement between the first and second stub shafts and the shaft drum. Each through-bolt may include threaded ends and a nut is provided at each threaded end to tighten the through-bolt and couple the first and second stub shafts to the shaft drum.

Further details and advantages of the present invention will become apparent from the following detailed description read in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an axial flow compressor showing the through-bolt arrangement according to the prior art;

FIG. 2 is a cross-sectional view of internal components of an axial flow compressor according to one embodiment of the present invention;

FIG. 3 is a detailed cross-sectional view of another embodiment of a pin in use with an axial shaft assembly in accordance with the embodiment of FIG. 2;

FIG. 4 is a detailed cross-sectional view of a pin in use with an axial shaft assembly shown in FIG. 2; and

FIG. 5 is an exploded view of internal components of an axial flow compressor in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the description hereinafter, spatial orientation terms, if used, shall relate to the referenced embodiment as it is oriented in the accompanying drawing figures or otherwise described in the following detailed description. However, it is to be understood that the embodiments described hereinafter may assume many alternative variations and embodiments. It is also to be understood that the specific devices illustrated in the accompanying drawing figures and described herein are simply exemplary and should not be considered as limiting.

Referring to the drawings in which like reference characters refer to like parts throughout the several views thereof, the present invention is generally described in terms of an assembly and method of attaching the stub shaft to the drum of the axial compressor rotor shaft. With reference to FIG. 1, an axial compressor 10 in accordance with a prior art embodiment is shown. Axial compressor 10 includes an inlet end 20 provided opposite a discharge end 30 along the axial direction of axial compressor 10. Inlet end 20 includes an inlet casing 40 for directing the working fluid into axial compressor 10. Desirably, inlet casing 40 is secured to a first end of housing 50 of axial compressor 10 by a plurality of mechanical fasteners 55. A discharge casing 60 is provided at discharge end 30 for expelling the working fluid once it is passed through axial compressor 10. Discharge casing 60 is secured to the second end of housing 50 by a plurality of mechanical fasteners 55.

With continuing reference to FIG. 1, an axial shaft assembly 70 is provided inside housing 50. Axial shaft assembly 70 is rotatable about a longitudinal axis 80 of housing 50 to drive the working fluid from inlet end 20 to discharge end 30. Axial shaft assembly 70 includes a plurality of rotor vanes 90 extending radially outward from its outer peripheral edge. A plurality of stator blades 100 extend radially inward from an inner peripheral edge of housing 50. Rotor vanes 90 and stator blades 100 are arranged in one or more alternating rows along longitudinal axis 80. Working fluid is redirected between the alternating rows of rotor vanes 90 and stator blades 100 as it is passed from inlet end 20 toward discharge end 30. A shaft seal 110 is provided at an interface with the inlet and discharge casings, 40 and 60, respectively, to seal axial shaft assembly 70 and prevent leakage of the working fluid from axial compressor 10.

Axial shaft assembly 70 further includes an inlet end stub shaft 120 and a discharge end stub shaft 130 connected to a hollow shaft drum 140. Inlet end stub shaft 120 and discharge end stub shaft 130 are connected to opposing ends of shaft drum 140 along its longitudinal axis. Shoulder structures 150 are provided at inlet end stub shaft 120 and discharge end stub shaft 130. Complementary shoulder recesses 160 are provided at each end of shaft drum 140 at the interface of shaft drum 140 with each of inlet end stub shaft 120 and discharge end stub shaft 130. Shoulder structures 150 and shoulder recesses 160 form a rabbet fit between stub shafts 120, 130 and shaft drum 140 to maintain the coaxial arrangement of these components. Stub shafts 120, 130 are further secured to shaft drum 140 via a plurality of through-bolts 170 extending from inlet end stub shaft 120 to discharge end stub shaft 130 through shaft drum 140. A plurality of holes 180 are formed in a radial pattern around the circumference of inlet end stub shaft 120 and discharge end stub shaft 130. Through-bolt 170 is inserted through each of the plurality of holes 180 to securely connect the stub shafts to shaft drum 140. Each through-bolt 170 must be pre-stretched in order to provide a large closing force in the rabbet fit between the stub shafts 120, 130 and shaft drum 140. This arrangement keeps shaft drum 140 from slipping relative to each of the stub shafts 120, 130 and prevents shaft drum 140 from axially and radially growing apart from the stub shafts 120, 130 during operation of axial compressor 10.

With the basic structure of axial compressor 10 having an axial shaft assembly 70 according to an embodiment of the prior art now described with reference to FIG. 1, a shaft assembly 200 will now be described with reference to FIGS. 2-5. In various embodiments, shaft assembly 200 is utilized with axial compressor 10; however, it is to be understood that this described embodiment is exemplary only, and that shaft assembly 200 may be utilized with a plurality of other turbine devices.

Referring to FIG. 2, shaft assembly 200 includes an inlet end stub shaft 210 and a discharge end stub shaft 220 connected to a hollow shaft drum 230. Inlet end stub shaft 210 and discharge end stub shaft 220 are connected to opposing ends of shaft drum 230 along its longitudinal axis 240. Shaft drum 230 may be manufactured from the same material as the stub shafts 210 and 220. Alternatively, shaft drum 230 may be manufactured from a different material compared to stub shafts 210 and 220. Shaft drum 230, desirably, has a plurality of recesses 250 for receiving rotor blades 260 (shown in FIGS. 4-5). Each recess 250 is provided on the outer surface of shaft drum 230. FIGS. 4 and 5, for example, illustrate a dove-tail recess 250; however, other embodiments are possible without departing from the scope and spirit of the structure of shaft drum 230. As shown in FIGS. 4 and 5, each rotor blade 260 is slidingly received inside recess 250, such that a plurality of rotor blades 260 is arranged in rows extending axially along the longitudinal length of shaft drum 230.

Referring back to FIG. 2, shoulder structures 270 are provided at inlet end stub shaft 210 and discharge end stub shaft 220. Complementary shoulder recesses 280 are provided at each end of shaft drum 230 at the interface of shaft drum 230 with each of inlet end stub shaft 210 and discharge end stub shaft 220. Shoulder structures 270 and shoulder recesses 280 form a rabbet fit between stub shafts 210 and 220 and shaft drum 230 to maintain the coaxial arrangement of these components. Shoulder structures 270 desirably extend over shoulder recesses 280 in a radial direction of shaft drum 230. This arrangement keeps shaft drum 230 from slipping relative to each of the stub shafts 210 and 220 and prevents shaft drum 230 from axially and radially growing apart from the stub shafts 210 and 220 during operation of axial compressor 10.

With continuing reference to FIG. 2, stub shafts 210 and 220 are further secured to shaft drum 230 via a plurality of pins 290 secured inside a plurality of keys 300 provided on the stub shafts 210, 220, and shaft drum 230. Keys 300 are disposed in a radial arrangement near the outer circumference of the stub shafts 210, 220 and shaft drum 230. Each key 300 includes a stub shaft slot 310 and a corresponding shaft drum slot 320 provided on the respective components. Each key 300 may be rounded in cross-section; however, one of ordinary skill in the art will recognize that other shapes, such as square or rectangular, may also be possible. Each pin 290 is shaped such that it may be disposed inside a corresponding key 300. For example, each pin 290 may have an elongated cylinder shape that corresponds to key 300. Desirably, each pin 290 is dimensioned slightly larger than a corresponding key 300 to allow for an interference fit between pin 290 and key 300. This arrangement keeps shaft drum 230 from slipping relative to each of the stub shafts 210 and 220 and prevents shaft drum 230 from axially and radially growing apart from the stub shafts 210 and 220 during operation of axial compressor 10.

With reference to FIG. 3, another embodiment of pin 290 is illustrated. In this embodiment, pin 290 includes a recess 370 extending axially inward from one end of pin 290. Recess 370 is desirably coaxial with a longitudinal axis of pin 290 and extends through a portion of the body of pin 290. Recess 370 is shaped to accommodate a pin insertion tool (not shown) for inserting pin 290 into corresponding key 300. In order to assemble pin 290 inside key 300 in an interference fit, it is desirable to cool pin 290 in order to slightly reduce its outer dimension. Such cooling may be accomplished, for example, using liquid nitrogen. Recess 370 is provided to facilitate handling of pin 290 as it is moved from a cooling station, such as a container with liquid nitrogen, and positioned for insertion into key 300. The pin insertion tool is inserted into recess 370 such that pin 290 may be cooled and inserted into key 300. The pin insertion tool is then retracted from recess 370. Recess 370 has a chamfered edge 380 to facilitate insertion of the pin insertion tool.

With reference to FIG. 5, an exploded view of shaft assembly 200 is shown. Optionally, a plurality of through-bolts 330 may be provided between inlet end stub shaft 210 and discharge end stub shaft (not shown in FIG. 5). Through-bolt 330 desirably extends through shaft drum 230 and includes threaded ends 335. A plurality of holes 340 are formed in a radial pattern around the circumference of inlet end stub shaft 210 and discharge end stub shaft 220. Through-bolt 330 is inserted through each of the plurality of holes 340 to securely connect the stub shafts 210 and 220 to shaft drum 230. A nut 350 is threaded onto each threaded end 335 of through-bolt 330 and tightened to couple the stub shafts 210 and 220 to shaft drum 230. A washer 360 may be provided at the interface between nut 350 and hole 340.

With the basic structure of shaft assembly 200 according to an embodiment of the present invention now described, a method of attaching stub shafts 210 and 220 to shaft drum 230 will now be described with reference to FIGS. 2-5. According to an embodiment of shaft assembly 200 for axial compressor 10, the method of attaching stub shafts 210 and 220 to shaft drum 230 begins at a step where inlet end stub shaft 210 and discharge end stub shaft 220 are provided on opposing ends of shaft drum 230. At the next step, shoulder structures 270 are provided at inlet end stub shaft 210 and discharge end stub shaft 220. At the following step, complementary shoulder recesses 280 are provided at each end of shaft drum 230 at the interface of shaft drum 230 with each of inlet end stub shaft 210 and discharge end stub shaft 220. Shoulder structures 270 and shoulder recesses 280 form a rabbet fit between stub shafts 210 and 220 and shaft drum 230 to maintain the coaxial arrangement of these components. Subsequently, a plurality of keys 300 are provided on the stub shafts 210, 220, and shaft drum 230. Keys 300 are disposed in a radial arrangement near the outer circumference of the stub shafts 210, 220, and shaft drum 230. At the next step, a plurality of pins 290 are inserted into one of the slots 310 or 320 formed on inlet end stub shaft 210 or shaft drum 230, respectively. Desirably, each pin 290 is dimensioned slightly larger than a corresponding key 300 to allow for an interference fit between pin 290 and key 300. At the final step, inlet end stub shaft 210 and discharge end stub shaft 220 are secured to shaft drum 230 by inserting and securing each pin 290 inside key 300. The interference fit between each pin 290 and key 300 is sufficient to prevent relative movement of the stub shafts 210 and 220 with respect to shaft drum 230. Optionally, a plurality of through-bolts 330 may be inserted through the corresponding plurality of holes 340 provided on the stub shafts 210 and 220 to further secure the shaft assembly 200.

While an embodiment of an axial compressor having a shaft assembly and a method of attaching the stub shafts to the shaft drum of the axial compressor is shown in the accompanying figures and described herein in detail, other embodiments will be apparent to, and readily made by those skilled in the art, without departing from the scope and spirit of the invention. For example, while the present disclosure generally discusses the shaft assembly 200 utilized with an axial compressor 10, it is contemplated that embodiments of the shaft assembly and method may be applicable to other gas turbine devices. The scope of the invention will be measured by the appended claims and their equivalents. 

The invention claimed is:
 1. A shaft assembly for an axial compressor, the shaft assembly comprising: a first stub shaft; a second stub shaft; a hollow shaft drum having a longitudinal axis, the first stub shaft and the second stub shaft coupled to opposing ends of the shaft drum along the longitudinal axis; a plurality of keys disposed in a radial arrangement proximate to the outer circumference of the first and second stub shafts and the shaft drum; and a plurality of pins engaging the plurality of keys, wherein the pins prevent the shaft drum from rotating axially relative to the first stub shaft and the second stub shaft and prevent the shaft drum from axially and radially separating from the stub shafts.
 2. The shaft assembly according to claim 1, further comprising: a plurality of shoulder structures provided on each of the first stub shaft and the second stub shaft; and a plurality of shoulder recesses provided at opposing ends of the shaft drum along the longitudinal axis, wherein the plurality of shoulder structures are operative for engaging the plurality of shoulder recesses to form a rabbet fit between the first and second stub shafts and the shaft drum for maintaining coaxial arrangement of the shaft assembly.
 3. The shaft assembly according to claim 1, wherein each pin is dimensioned to be larger than the corresponding key to allow for an interference fit between the pin and the key.
 4. The shaft assembly according to claim 1, wherein each key comprises a slot formed on the first and second stub shafts and a corresponding slot formed on the shaft drum.
 5. The shaft assembly according to claim 1, wherein each pin includes a recess substantially coaxial with a longitudinal axis of the pin, the recess adapted for facilitating insertion of the pin into the corresponding key.
 6. The shaft assembly according to claim 1, further comprising one or more through-bolts extending axially between the first and second stub shafts through the shaft drum, the one or more through-bolts providing a closing force to prevent relative movement between the first and second stub shafts and the shaft drum.
 7. The shaft assembly according to claim 6, wherein each through-bolt includes threaded ends and a nut is provided at each threaded end to tighten the through-bolt and couple the first and second stub shafts to the shaft drum.
 8. An axial compressor comprising: a housing having an inlet casing provided at one end and a discharge casing provided at an opposing end; an axial shaft assembly rotatably disposed inside the housing, the axial shaft assembly extending between the inlet casing and the discharge casing along a longitudinal length of the housing; a plurality of stator blades extending radially inward from an inner surface of the housing; a plurality of rotor blades extending radially outward from an outer surface of the axial shaft assembly, wherein the axial shaft assembly comprises: a first stub shaft; a second stub shaft; a hollow shaft drum having a longitudinal axis, the first stub shaft and the second stub shaft coupled to opposing ends of the shaft drum along the longitudinal axis; a plurality of keys disposed in a radial arrangement proximate to the outer circumference of the first and second stub shafts and the shaft drum; and a plurality of pins engaging the plurality of keys, wherein the pins prevent the shaft drum from rotating axially relative to the first stub shaft and the second stub shaft and prevent the shaft drum from axially and radially separating from the stub shafts.
 9. The axial compressor of claim 8, further comprising: a plurality of shoulder structures provided on the first stub shaft and the second stub shaft; and a plurality of shoulder recesses provided at opposing ends of the shaft drum along its longitudinal axis, wherein the plurality of shoulder structures are operative for engaging the plurality of shoulder recesses to form a rabbet fit between the first and second stub shafts and the shaft drum for maintaining coaxial arrangement of the shaft assembly.
 10. The axial compressor of claim 8, wherein each pin is dimensioned to be larger than the corresponding key to allow for an interference fit between the pin and the key.
 11. The axial compressor of claim 8, wherein each key comprises a slot formed on the first and second stub shafts and a corresponding slot formed on the shaft drum.
 12. The axial compressor of claim 8, wherein each pin includes a recess substantially coaxial with a longitudinal axis of the pin, the recess adapted for facilitating insertion of the pin into the corresponding key.
 13. The axial compressor of claim 8, further comprising one or more through-bolts extending axially between the first and second stub shafts through the shaft drum, the one or more through-bolts providing a closing force to prevent relative movement between the first and second stub shafts and the shaft drum.
 14. The axial compressor of claim 13, wherein each through-bolt includes threaded ends and a nut is provided at each threaded end to tighten the through-bolt and couple the first and second stub shafts to the shaft drum.
 15. A method of attaching stub shafts to a shaft drum of an axial compressor, the method comprising the steps of: providing an axial shaft assembly comprising: a first stub shaft; a second stub shaft; and a hollow shaft drum having a longitudinal axis, the first stub shaft and the second stub shaft coupled to opposing ends of the shaft drum along its longitudinal axis; providing a plurality of keys disposed in a radial arrangement near the outer circumference of the first and second stub shafts and the shaft drum; and providing a plurality of pins engaging the plurality of keys, wherein the pins prevent the shaft drum from rotating axially relative to the first stub shaft and the second stub shaft and prevent the shaft drum from axially and radially separating from the stub shafts.
 16. The method of claim 15, further comprising the steps of: providing a plurality of shoulder structures provided on the first stub shaft and the second stub shaft; and providing a plurality of shoulder recesses provided at opposing ends of the shaft drum along its longitudinal axis, wherein the plurality of shoulder structures are operative for engaging the plurality of shoulder recesses to form a rabbet fit between the first and second stub shafts and the shaft drum for maintaining coaxial arrangement.
 17. The method of claim 15, wherein each pin is dimensioned to be larger than the corresponding key to allow for an interference fit between the pin and the key.
 18. The method of claim 15, wherein each key comprises a slot formed on the first and second stub shafts and a corresponding slot formed on the shaft drum.
 19. The method of claim 15, further comprising the step of providing one or more through-bolts extending axially between the first and second stub shafts through the shaft drum, the one or more through-bolts providing a closing force to prevent relative movement between the first and second stub shafts and the shaft drum.
 20. The method of claim 19, wherein each through-bolt includes threaded ends and a nut is provided at each threaded end to tighten the through-bolt and couple the first and second stub shafts to the shaft drum. 